1. Field of the Invention
The present invention relates generally to rotational molding methods and particularly to a rotational molding which create a shiny and hard surface layer for a molded part.
2. Description of Related Art
Rotational molding, which is often referred to as "rotomolding", is a process where a powdered thermoplastic is poured into a mold and the mold is suspended in an oven. The material utilized for the powdered thermoplastic is often polyethylene, while the mold is ordinarily constructed from metal. Once suspended in the oven, the metal mold is heated beyond the melting point of the powder and rotated to evenly coat the inner surface of the mold with the melted plastic. Once properly heated, the mold is then cooled and the part is demolded.
The mold serves two purposes: (1) to transfer heat to the resin to establish the shape to be formed and (2) to provide a cavity in which to cool the material. Typically, a pre-measured resin is loaded into the mold and the mold is disposed within an oven and rotated on both its vertical and horizontal axis. As the resin melts it sticks to the inner surface of the mold, coating such surface evenly. Usually, the rotation of the mold continues during the cooling cycle to allow the intended part to maintain an even wall thickness. Once the part is cooled, it is removed from the mold.
Rotomolding has been utilized, amongst other items, for the manufacture of boats. Some advantages to rotomolding, as compared to other molding techniques is the relatively low cost for molds, ease in adaptation for short production runs, the elimination of secondary tooling and the minimal amount of scrap which is produced from the process. Rotomolding also provides for consistent wall thickness and sharp outside corners which are essentially stress free.
Rotomolding has also been traditionally limited to use with a single material. Thus, one specific problem with rotomolded products is that the surface of the product is soft.
When creating a structure comprise of at least two different materials, bonding of the materials is typically an issue, particularly with materials having disparate properties. In the past, rotomolding processes have not been used for marrying materials having separate properties.
It is therefore to the effective resolution of the aforementioned problem that the present invention is directed.